Microelectronic technology is the foundation for many revolutionizing technologies such as telecommunications, personal computing and communications, and medical instruments. Semiconductor wafers made of various semiconductor materials (e.g., Silicon and Gallium Arsenide) form the backbone of the ever growing microelectronics industry. They are widely used for fabricating discrete electronic devices, integrated circuits, microelectromechanical devices and systems, and opto-microelectronic devices such as light-emitting-diodes and laser diodes.
Characterization of semiconductor wafers and processed substrates is an indispensable component of microelectronic technology. Better measuring, characterizing, and controlling substrate material properties can improve the performance of semiconductor devices and circuits. Hence, such test methods can have a significant impact on both the technical and economic aspects of the semiconductor manufacturing industry. Substrate characterization not only greatly facilitates research and development in new semiconductor wafer materials and devices, but also provides a critical tool in increasing the throughput of wafer inspection prior to shipping, providing needed information on wafer quality for improving manufacturing and application implementation.
One critical area of semiconductor characterization is substrate evaluation and specification in terms of substrate resistivity, substrate absolute thickness, and thickness uniformity across a substrate. This operation is essential to failure analysis and device performance. There has been continuous effort in developing various reliable and fast techniques of substrate characterization, particularly, techniques that can work with ultra-thin wafers (e.g., 1 .mu.m.about.100 .mu.m in thickness) and most importantly, nondestructive measuring techniques.
A number of techniques are known to measure important substrate parameters including the resistivity, absolute thickness, and thickness variation of a semiconductor substrate. However, many of these techniques have limitations. A majority of existing characterizing systems for semiconductor substrates cannot nondestructively measure all three of the above parameters simultaneously.
A mechanical micrometer is an example of a technique that physically touches a wafer in order to measure the thickness. Touching can cause damage or scratches on the wafer that adversely affect the performance of a semiconductor device made from that wafer. Various nondestructive techniques, such as capacitance probes and sonar probes, have been developed to avoid this problem. However, many of these devices are unable to measure thin wafers which have an absolute thickness less than 100 .mu.m. In addition, calibration standards are typically required in the operation of these systems.
Several devices using optical methods (e.g., light reflection and interference) have been employed to measure dimensions of a workpiece. However, these devices usually are not suitable for measuring thin wafers.
Furthermore, many of the conventional techniques for measuring wafer thickness cannot nondestructively and directly measure and view localized features in a micro machined substrate. Such capability is important to microelectromechanical device manufacturing industry.